Electrohydraulic servo valve



Sept. 20, 1960 G. K. REEN ErAL ELECTROHYDRAULIC sERvo VALVE 5 Sheets-Sheet 1 Filed Oct. 30, 1959 w l G Sept. 20, 1960 G. K. REEN ETAL ELECTROHYDRAULIC sERvo VALVE Filed Oct. 50, 1959 zO-PFEOXM EEE :50am m wiel Esca 25221 I 62:02u l 8.5282 I ...IIIII sept. 2o, 1960 Filed Oct. 30, 1959 5 sheets-sheet 3 RETURN R 5156/2/36 P/LOT STAGE DRIVER POWER RETURN l wm v P0 50u/m5 e? fg, Fs 5? e? POWER (0l/TPU OUT/UT 6em/d K. Ree/7 United States Patent 2,953,123 ELECTROHYDRAULIC sERvo VALVE Gerald K. Reen and .lohn A. Trevett, Orange, Conn., as-

signors to Textron Electronics, Inc., Providence, RJ., a corporation of Delaware Filed Get. v30, 1959, Ser. No. 849,951

4 Claims. (Cl. 121-157) This invention is for improvements in electrohydraulic servo valves adapted to convert direct current hydraulic energy into alternating hydraulic energy to be used in a vibration exciter. Itis a valve adapted to cause the actuator of the exciter to vibrate in response to a low level electrical signal input and to control the amplitude and frequency of vibration. One object of the invention is to increase the strength of the exciter without increasing the electric signal or sacricing the range of frequencies over which `the device can be used. This is achieved by means which are simple and easy to construct. Our invention is related to the construction of a spool type servo valve.

In the drawings Fig. l is a schematic view of an electrohydraulic servo valve and exciter actuator of the prior art showing a three land spool valve at the power stage, the clearance around the Iands being much exaggerated.

Fig. 2 is a block diagram ofthe servo valve connections, electrical, hydraulic and mechanical, for a hydraulic vibration exciter suchi as in Fig. l whenV modiiied according to our invention.`

Fig. 3 is a schematic diagram of the piping. connections for our novel servo valve using 'a live land power spool and suitable for use in Fig. 2.

Fig. 4 isa view in vertical cross section across the electrohydraulic servovalve of an exciter embodying the servo Valve shown schematically in Fig.Y 3. It shows'the pilotjstage in the center and the power stage at theright'. lFig. 5` is a'view in vertical longitudinal section through the structure of- Fig. 4 takenY at 90 removed from the sectional view'of Fig. 4.`

Fig. 6 Iis a perspective diagram of aV ve land spool i according to the invention.

Fig.V 7 is a graph comparing the flow at diiferent frequencies of the conventionalY three landspool-valve, and three landy spool with reduced-r end area and the tive land spool. u

In devices of the sort to whichV this invention relates asmallor low level electric signal is used to operate and control the device; As shown in Pig. l an electrical in'- strument, such for example as an electrodynamic driver l isusedto displace longitudinally the spool 2. of a spool valve, which` spool. is` restrained by a. spring 3: atvthel opposite. end of thespoolfromithe driver 1 .Y Thiscon'- verts'. the alternating electric currentin. the driver into' alternatingmovement of the spool. y,

The embodiment we are about to describe is of a valve. which has both. a pilot stageA and' a power stage but it shouldl be understood that the invention. can be' used. to.4 advantage in a servo valve-thathas only one hydraulic stage. This single stage construction would have many of the advantages` obtained with the embodiment that we have chosen and which contains both a pilot stage and a power stage. In the case of al single stagev valve which we will call the pilot stage, that would have the iive landgconst'ruction sh'own` inthepowe'r. stage .i Stage.

ample, the pilot stage by itself has the advantage of producing vibrations of high frequency. What we have here-4 inafter termed the gain and hence the frequency band width of the conventional pilot stage can be increased by approximately a factor'of 2 with our live land system and the factor of course could be increased still higher if more lands are employed. This is true in spite of the fact that the pilot stage is driven by the electro` dynamic driver rather than by hydraulic iiuid.

The immediate objective is to connect the` sides of the exciter piston alternately to a high pressure source and then to a low pressure exhaust. If the ilow of oil out of the pilot stage through its connections is not strong enough for the desired size of exciter, a power stage spool type valve has to be added in series to the pilot The embodiment which we have chosen to be shown and described uses the conventional three land pilot stage and a five land power stage constructed as hereinafter set forth.

The pilot stage spool 2 has three lands of which a center land 4 is adapted to be connected to an hydraulic power source P. When the electrodynamic driver moves the spool to the right from the position showri in Fig. l, oil flows past the left end 5 of the center yland and into a connection 6 leading to the left end of the power stage valve hereinafter described. This connection 6 at the pilot stage is located between the center landv 4 and the land 7 at the left end of the valve. There is a similar land 8' at the right end of the pilot' valve spool and a connection 9 between the center land 4 and end land 8 at the pilot stage and the right end of the' power stage valve. The -two end lands 7', 8 are kcort-Y nected to an exhaust or low pressure return R. Thus' the pilot stage alternately connects the ends of the power stagel to .the pressure P and the return R. The' casing 10 for the power stage spool 11 has been shaped for three lands 12, 13, 14 arranged in corresponding manner to the pilot stage. The center land 13 is adapted to permit admission of oil from the same pressure source P as the pilot stage center land 4 when the sp'o'ol 11; is displaced. The end lands l2 and`14 were' for the 10W pressure return R. The pressures P and returns R fron the two stages can be united respectively at the pressurepump 15 with which they operate (see Fig. 2). The ow from the pilot stage comes to the power stage through connections 6, 9 to the ends 18, 19 of the power stage of'tlie embodiment `hereinafter describedibut'wouldhave V.

many of'tlie advantages of theinvention." for exi spool. l

The oil which ows through the pilot stage displace's" the power stage spool. When displaced to the right the source of pressure P is connected to the left end 20 of Vthe actuator 21 of the exciter and the return tothe right end 22 of'. the actuator. Displacement of the power stage `to the left reverses these connections. The con= nections from the power spool 1d to the actuator ends 20 and 22 are indicated in Fig. l at 23 and 24. Each connection is attached to the casing 10 of -the power stage between the center land and the land at its end ofthe spool and -is attached to the cylinder 25 for the actuator 21. The actuator has a piston as usual. The table carrying the test item isl on one end of the actuator.

In order to maintain the power'spool 1l in its neutral position a transducer 26 is attached to one end of the spool. The reciprocatory movement of the spool produces an electrical signal proportional to the displacement of the spool. As shown in Fig. 2 this s-ignal isutilized in a conventional feed backV circuit'. The excitation for the transducer 26 is obtained from an oscillator 31. The feed back signal is summed up in the amplifier at 29, and is then raised in power level in order to center the power valve. In the same way transducer 30 positions the actuator 21. Thus the system is controlled Ywith'the electrical-signals while the amplitude and frequency of vibration is varied.

Turning now to the need of increasing hte power of the exciter starting from the old low level electrical signal, it should be recognized that the owout of the power stage is determined bythe orifice opening which results when the power spool is displaced. VBasically an electrohydraulic servo valve produces a ow output Q for a given input current I, into the device. With standard techniques, Van output ow of 2Q0 can be obtained Vfor a given input current I. Unfortunately, however, this is obtained at the expense of frequency response such that the new output 2Q0 can be obtained only at a lower frequency of F2 than the old frequency F, obtained with output Q0. It can beV shown that the diameter of the power spool d2 determines the output flow Q0 as well as the flow from the pilot stage qd which is necessary to drive the power spool. The gain of the power stage can be dened as It has heretofore been suggested how the area of the lands in the power spool can be increased but the amount of output required from the pilot stage keptrat its original figure. This was to be'done by reducing the effective area of the ends on the power stage spool. The surfaces exposed to the oil from the pilot stage were sealed off from the rest of the spool. r[his is shown inthe Carson Patent 2,790,427. We have found that this not only creates certain manufacturing difficulties but affects performance in a number of ways hereinafter explained. 'I'his method of maldng the spool diameter at the ends smaller than the lands is undesirable from the manufacturing point of View because of the close tolerances it requires to maintain the necessary concentricity and slid- -ingseals between the two diameters on the mating pieces. j To determine more clearly what the nature of the problem was, and in a practical way, we calculated Y. Y Y Y Y qa asfollows: Y Y Y -Let Q0, the llow to the actuator, equal an oriiice constant times the area of a single orice times the square root of the head of fluid dropped across a single orifice- Let the area of a single orifice A0 equal 3.1416 times the diameter d2 of the power spool times the spool displacement single amplitude'- f In order to maintain the stress and deliectionrin the spool shank within limits we establish- Ay=oil ow area between spool lands.

YIn order to prevent flow saturation of the valveat maximum AYstroke (X2) therarea AF should be at least three timesA0n1aximum.Y

Yeo

Substitute and solvefor 210 gives Y and Y j t Q0=1f 13 CvhX22 Determine tlow required to drive spool qd.

t VA study of these last equations indicates that there are only three ways to increase theow capacity of the' power stage Q0. One is to increase the head h of nid dropped across a single orice, another is to increase the usable spool displacement X0',V and the third is to increase the spool diameter d2. YSinceh has practical limits, vX3 and d are the most important variables. -v Further, the power stage gain varies inversely with the frequency, the usable spool displacement X2,' and Ythe spool diameter d'2.'A lI n other words, vifY one'seeks to extend the frequency range or the owV capacity Aof the power stage, the gain in the valve reduced: and*consequently` the vpilot "stage requirements increase rapidly. lIY-Iowevensince the basic problem underlying the points' aboveflay in the fact, that both the ow capacity out of the' power stage andthe flow required .to drive theV power spool are determinedby the spool diameter, our practical solution to the problem, -is to increase Vthe number lof metering surfaces Von a given spool Without changing the spool diameter by connecting the ports from the lands to theactuator in parallelas shown in Fig. 3. We have added center lands. g Y x i f We found that thecommon construction of a spool type valve contains four active metering surfaces at the power stage. IIhese four surfaces meter the hydraulic ow from the'p'ressure source `to the actuator and the corresponding ow to the return. These four metering surfaces can rbe arranged fby designing a conventional spool with either three or four lands. The arrangement of the flow passages remains the same in bothrcases, thereby retaining the same ow pattern. In the three 76 land spool designs shown n'Fig. 1 the .ow ,is metered at the pressure port yand at the return ports. In a four land spool design the ilow isrmetered at'the two load l YIn, comparison to these prior art powerV stage designs four metering surfaces, our new ive land constructionineorporates a total-foffeightj metel'llgsurfaccsl assenza t should be noted that these eight metering surfaces are obtained -by adding only two additional lands to the common three land design (see Fig. 3). The lands are numbered 33, 34, 35, 36 and 37. Thus we do not increase the number of lands by the gain factor of 2. Only the number of land ends to lwhich metering functions were `assigned is increased by that factor.

The flow pattern of our iive land embodiment is such that the two pressure passages PS come in parallel from the same pressure source to the power spool, and the three return paages PR going lback to the pump 15 (Fig. 2) are in parallel. These live passages Iare arranged in alternation PR, PS, one for each land (Fig. 3). Between every two lands there is an output port 32. The rst and third ports 32 are connected by parallel passages 3S to output 1. The second and fourth ports 32 are connected by parallel passages to output 2. Output -1 goes to one side of the actuator piston and output Z -to the other. It will be seen that the output passages are in parallel or -tandem but all the spool lands are voperated from the same input source, i.e. series. Also the lands in series are not double in number but the gain of the power stage is doubled.

It will bev seen that the lands of the spool are physically connected in series for operation by the ow from the pilot stage while the lands feedoutflow to the actuator piston by connections or passages in parallel.v

The showing of the pilotl stage of our invention, Fig. 4, dlfers from the diagram of Fig. vl in that the spring 3 is replaced by a resilient cushion 40.

The construction of the embodiment of Figs. 3, 4, 5 `and 6 having been described, its operation will now be set forth in so far as it concerns the servo Valve directly. Assuming that an electrical impulse is received -by the electrodynamic driver 1, the pilot stage spool 2 will be pushed to the .right in Fig. 3 and downwardly in Fig. 4 against the resilient cushion 40. With the spool 2 in this position the fluid connection 6 from the pilot stage to the power stage will be opened past the left end of the center land 4 of the spool and the return for the exhaust will be connected to the fluid passage 9. As -a result of the relatively high pressure on the left end 18 of the power spool and the relatively low pressure on the right end 19 of that spool, this latter spool will also be moved to the right at the same -time as the pilot spool. This will open the metering surfaces at the left ends of the lands 34 and 36. This movement also causes the left end land 33 to shut ofrr the left hand return connection PR and open the middle and right hand PR oonnections by movement of the middle land 35 and the right hand end land 37. I-t will be seen that the power stage therefore yhas passages connecting the lands to the exciter in parallel, while from the point of rView of the ow from the pilot stage the lands are .in series. This gives us the increased output flow and the desired gain at the power stage.

Electrohydraulic driver 1, aided by the cushion 40, moves the pilot spool to the left, the center land 4 brings into action the connection between the pressure source and the connection 9 to the right end of the power stage spool. Therefore the power Stage spool 16 will also move -to the left. While Ithis does not open the return connection PR at the right of Fig. 3, the right end `of the four remaining lands, namely, 33, 34, 3S and 36, all open por-ts as follows: The lands 34 and 36 open the ports 32 leading to the parallel connections 39 to output 2 to the pressure source PS while keeping their connections to the return closed while the right ends of lands 33 and 35 are open to the return connections PR. Thus output 1 is connected in parallel to two exhaust passages and output 2 to two pressure ows. This will result in having the actuator 21 move in the opposite direction from the one occupied when the two spools were at the right and the reciprocation or alternation is thus maintained under the direct crmtrolv of driver 1. ,i

The power spool neutral position shown in the drawL ings is maintained by the transducers 26 land 30f working' with Ithe driver and the other electrical elements shown land described above. :s

In order to summarize the capabilities and` advantages of Iour invention we Yhave supplied a-s Fig; 7 a grapli showing the comparative flows obtainable at different fre*-V quencies from the conventional three land spool valve; the three land vspool valve with reduced end area'and our five land spool valve. The major improvements in performance of our iive land spool over the others are `as follows: v Y l' (l) Two times the output ow can -be rachievedby the ve land design in the region of usable spool stroke (see Fig. 7). y;

(2) The maximum output ow that can be obtained from the conventional three land spool can be obtained from the five land spool to two times the maximum fre', quency to which this ilow can be obtained for the con.-V ventional three Iland spool. l Y Y (3) rPhe weight of the iive land spool has increased by a factor of 5 3, while the end area has remained-coriL stant. The weight of the reduced end area three land spool has remained constant, but lthe end area has come down by a factor ofV 2. Therefore, the ratio of weight to end area is less. for the live land spool thanfor the reduced end area th-ree land spool. This means that'the five land spool will operate to higher frequenciesVV before encountering the region of input pressure limit. ('See Fig. 7.)

(4) Because of the reduction in end area of the three land spool the natural frequency of the spool on its input oil column is lower by a larger factor than that of the five land spool on its input oil columns. This means that the ultimate cut-olf frequency is higher for the five land spool than for the reduced end area three land spool.

(5) The curve shows the approximate theoretical performance to be expected. However, additional dynamic forces that are introduced in providing for an end area seal in the reduced end area design, will serve to further limit the frequency response of this device above the region of usable spool stroke.

(6) The manufacturing difficulties associated with providing for a reduced end area spool requires additional alignment problems, sealing problems, and lapping problems that are not encountered in the iive land design.

The invention achieves power stage gain without loss of frequency. It is also advantageous in terms of its simplicity, its manufacturing costs and in that it does not introduce serious new problems associated with the operational or dynamic forces on the spool. Also the weight of the spool itself does not increase as rapidly with an increase in ow output as would be true if the increase in ow output were due to increase in the diameter of the spool. Thus our five land construction doubles the flow capacity but only increases the mass by 60 percent. A servo valve made according to our invention could have veseven or nine, etc. lands.

What is claimed is:

l. An electrohydraulic servo valve for a vibration exciter with an actuator piston and powered by a direct current hydraulic power source, comprising a valve having one or more spools, there being at least iive spaced lands on one spool adapted to convert direct into alternating hydraulic power, low level electrical means to control reciprocation of the spools, parallel passages connecting one side of the actuator piston to alternate spaces between the lands on said tive land spool, and similar parallel passages connecting the other side of the actuator piston to the intervening spaces between said lands, in combination with flow orifice metering surfaces at the ends of said lands which face spaces between them, other passages from and to the direct current power source adapted to be connected to the metering surfaces selectively as the spool reciprocates; whereby there is a gain in the power trans# mitted to the exciter without lowering the frequency of vibration. Y W L YAn electrohydraulic servo valve for a vibration exciter with an actuator piston` and'powered by a direct current hydraulicV power source, comprising a low level electrical input means, a pilot stage spool valve Ydriven by. the electrical input means and adapted to convert direct current hydraulic power from Vthe power source into alternating hydraulic power,V and a power stagey spool vlvefhavng at least ve spaced lands driven by the alternating hydraulic power, parallel passages -connecting one side of the actuatorY piston to alternate spaces between the lands on said iive land spool,Y andsimilar parallel passages connecting the other side of the actuator piston to the intervening spaces between said lands, in combination with ow orifice metering surfaces at the'ends of said landsfwhich'face'spaces between them, other passages from and .to the direct current power source adapted to be connected to the metering surfaces selectively as therspool reciprocates; wherebyV there is a gain in the power transmitted tothe exciter by thev power stage valve over the pilot stage valve without limiting the frequency of vibration.V

3. An electrohydraulic servo valve according to claim 2 :in which there are connections'from the pilot spool valve applying the alternating hydraulic energy from the pilot spool valve at the outside end area of the two end lands of the power stage valve using the maximum electve area of theends of said lands Ywhereby the increasein thenumber of metering orifices driven in series by the flow applied at the` ends of the spoolrcauses gain in thetoutllow'controlled by the spool. f

` 4.. L An v`electr,olivdraulic servo valve, fora; vibration ex-g tei'lV with an "actuator" piston' and powered by direct "cur-V` rent Vhydraulicpower; comprising a spool .valve With'one Y or more spools having at least live spaced lands on one spool, said'ive land spool being adapted to convert the direct into alternating hydraulic power, low level electrical means to vreciprocate one ofthe spools, parallel passages connecting'oneside of the actuator piston to alternate spaces between therlands on saidve land spool,Y and similarfparallel passages yconnecting the other side of the actuator piston to intervening spaces between said lands, in combination with other passages connected 'opposite the lands from and to the direct 'current hydraulic power; andorice metering surfaces'at each end of each land except the outside ends of the end lands adapted to conf. nect the power passages to the actuator piston passages as said spoolreciprocates; whereby the weight of said spool is less in proportion than the gain in output ow. p

References Cited in the file of this patent i 

